Generally speaking, an electric machine comprises a stationary part, commonly referred to as “stator” (or “stator assembly”), and a mobile part, both equipped with windings of electrical conductor and/or sources of a magnetic and/or electromagnetic field. Together with the machine structure, these windings and sources always form both an electric circuit (defined as an assembly of structures and materials with an electric current and/or an electric field flowing through it) and a magnetic circuit (defined as an assembly of structures and materials with a magnetic field flowing through it). In order to operate, the electric machine uses electromagnetic induction (produced by the linkage of magnetic field fluxes with the electric windings) and/or electromagnetic forces (generated by the magnetic/electromagnetic field sources on the electric windings with current flowing through them and/or on the other magnetic/electromagnetic field sources). Some electric machines (for example, electric motors) can convert the electric current circulating in the electric windings into movement of the mobile part relative to the stator. Other electric machines (for example, generators) can generate electric current and/or electrically driving forces in the electric windings using the motion of the mobile part relative to the stator. An electric machine of this kind can normally be used in both ways (that is, as a generator and as a motor). The electric windings can be made around a core of magnetic material in order to optimize the effect of the linkage of the magnetic flux with the electric windings themselves.
In one type of electric machine, the mobile part is a rotary member, also known as “rotor” (or “rotor assembly”). The axis of rotation of the rotor is particularly important and is usually used as the reference and/or symmetry axis for the structure of the electric machine. As the rotor moves relative to the stator, portions of the magnetic field sources and portions of the electric windings face each other at a given distance defining a gap between the rotor and the stator. There is a geometrical relationship between the axis of rotation of the rotor and the way in which the streamlines of the magnetic field, generated by the sources, are arranged in the gap between the stator and the rotor. Based on this geometrical relationship, machines of this kind can be divided into two categories: radial flux electric machines and axial flux electric machines. In a radial flux electric machine, the arrangement of the magnetic field sources and of the electric windings, with which the magnetic field is linked, is such that, in the aforesaid gap between rotor and stator, the streamlines of the magnetic field can be approximated with segments stemming from straight lines that are perpendicular to the rotation axis of the rotor and are arranged in a radial manner relative to the rotation axis itself. In an axial flux electric machine, the arrangement of the magnetic field sources and of the electric windings, with which the magnetic field is linked, is such that, in the aforesaid gap between rotor and stator, the streamlines of the magnetic field can be approximated with segments stemming from straight lines that are parallel to the rotation axis of the rotor.
In general, an electric machine of this type comprises: a stator with a generically toroidal shape and a rotor, which are both coaxial to the rotation axis of the rotor.
In radial flux electric machines, the rotor is generally inserted into the central hole of the toroid making up the stator. In the most common type of axial flux electric machine, the rotor has the shape of a disc and faces one of the bases of the toroid making up the stator. In some cases the machine has two disc-shaped rotors, each one facing a respective face of the stator. Some electric machines can comprise two or more toroidal stators, which alternate with disc-shaped rotors (in a particular configuration of this type, a disc-shaped rotor is arranged between two toroidal stators).
Generally, on the rotor there are placed the magnetic field sources, which are preferably made with permanent magnets, whereas on the stator assembly there are usually placed the electric windings with which the magnetic field is linked. The magnetic field sources are usually placed on a circular crown of the rotor disc, which faces a base of the toroid making up the stator.
The stator of an axial flux electric machine, in particular, comprises a core with a toroidal shape, which is provided with an outer cylindrical lateral surface and with an inner cylindrical lateral surface, which are both coaxial to an axis that coincides with the rotation axis of the rotor. The core is also delimited, along the axis, by a first and a second base. The core is made of a magnetic—preferably ferromagnetic—material. The electric windings are manufactured in the form of a plurality of coils that are arranged one after the other at a given distance along the annular shape of the core and are electrically connected to one another in different ways. Each coil usually has a through hole, around which the electric conductor making it up is wound.
The bases of the core can consist of flat surfaces, in which case the core of the stator is defined as “slotless”.
Alternatively, either or both bases of the core can have protuberances, also called “teeth”, which project from the core along the common axis of the cylindrical lateral surface covering a given distance. In general, the teeth extend in length from the outer lateral surface up to the inner lateral surface. The teeth can be manufactured as one single piece together with the core or be fixed thereto in different ways after the core has been manufactured. The space defined between two successive teeth in the annular shape of the core is also known as “slot” and usually houses portions of the windings. In this case, the core of stator is “slotted”. The teeth are usually made of a magnetic material and help link the magnetic flow with the windings (in particular by affecting the magnetic reluctance of the magnetic circuit in correspondence to the electric windings).
In order to reduce parasitic currents in the core (currents that tend to arise in the core along rings surrounding the streamlines of the magnetic field and determine efficiency losses of the electric machine), the core itself is usually manufactured by winding a metal sheet on itself in a spiral shape around the common axis of the cylindrical lateral surfaces of the core. In this way, the interfaces between a metal sheet winding and the other are distributed crossways relative to the annular paths of parasitic currents, thus breaking them and reducing their influence.
In a first stator configuration, the solid part of the core goes through the through hole of the coils. Therefore, in general, when the core is provided with slots, one of the teeth is arranged between one coil and the other and each coil rests on a plane of its that is transverse to the toroidal shape of the core.
In a second stator configuration provided with slots, a corresponding tooth goes through the through hole of each coil. Hence, the coil is entirely arranged on a base of the core or on the other base of the core and part of its electric conductor is inserted into the slots between the teeth. The same slot can be shared by two consecutive coils along the annular shape of the core, or not.
In both stator configurations described above, the coils project in a radial direction, relative to the axis, towards the outside of the stator. Therefore, on the outside of the rotor, in a radial direction relative to the axis, a space is defined between two consecutive coils.
The electric machine also comprises a casing (or case), which is generally fixed to the core with the windings and surrounds at least the core around its axis. In general, the casing is part of the stator of the electric machine.
During the operation of the electric machine, power losses occur in the electric circuit and in the magnetic circuit, namely:                the so-called “cooper losses” (namely power losses in the electric circuit of the machine due to the Joule effect mainly caused by the current flowing in the different windings and electric conductors);        the so-called “iron-losses” (namely power losses in the magnetic circuit of the machine mainly due to the magnetic hysteresis of magnetic materials and to parasitic currents—also known as “eddy currents”—which occur in the active parts of the machine, in particular in the stator parts, i.e. core and coils).        
These power losses generate the development of heat, which must be removed and moved to the surrounding environment as effectively as possible: the development of excessively high temperatures in the active parts of the machine (core and coils, in the case of the stator) might jeopardize the integrity and the functionality of the electrically insulating parts, which are the most delicate in terms of temperature.
U.S. Pat. No. 7,332,837 B2 discloses a stator for an electric machine with an attached cooling system. More specifically, the stator assembly comprises a toroidal core. Electric conductor coils are arranged along the annular shape of the core so as to be spaced apart from one another, each coil having a lying plane that is arranged radially, i.e. contains the axis of the toroid. The stator assembly comprises an outer casing made of metal (preferably aluminium), which surrounds the core from the outside, remaining coaxial to the core itself. The casing has teeth that radially project inwards and are each inserted in the space between two successive coils along the annular shape of the core. The body of the annular casing comprises, embedded therein, a cooling duct, which also has an annular shape and circumferentially and externally surrounds both the core and the coils. A cooling liquid flows in the cooling duct. The teeth of the casing act as cooling fins. The spaces between these cooling fins and the coils are preferably filled with a filling material having a good heat conductivity.
The stator of the electric machine with cooling system described above has some drawbacks.
In particular, the cooling of the body of the coils is mainly entrusted to the sole contact with the metal teeth of the casing, the annular cooling duct surrounding, from the outside, the entire assembly consisting of the core and the coils. For this reason, the cooling obtained in this way is not optimal and risks being insufficient in particular operating conditions or in electric machines that have to be optimized to ensure high performances.
International patent application PCT/IB2009/007570 (published with no. WO 2010 061278 A2) discloses the stator of an electric machine provided with a liquid cooling duct, wherein the cooling duct is applied, according to an annular arrangement, around the core and comprises a plurality of sections, which are oriented parallel to the axis of the stator and are each inserted between two successive coils along the annular shape of the core, into the space created between the parts of the coils that radially project from the core. These sections of the duct are connected to one another by duct portions that circumferentially develop around the core.
By leading the cooling liquid flow directly between the coils of the electric windings, this system ensures a good cooling efficiency. However, this solution does not lack drawbacks.
In particular, the cooling duct, which is preferably manufactured in the form of a shaped serpentine, is conceived as an element on itself, which is applied onto the core and then locked there by applying an outer containing casing made of metal and/or by subsequently applying a suitable resin made of an electrically insulating material with good thermal conductivity properties. For this reason, the stator is difficult and partially delicate to assemble, especially when the duct and the core have to be coupled and when the entire assembly has to be closed.